Optical coherence tomography is still a very new imaging technology. An outline of optical coherence tomography is given in, for example, the article “Optical coherence tomography for ultrahigh resolution in vivo imaging” by James G. Fujimoto, in “Nature Biotechnology”, Volume 21, Number 11, November 2003, pp. 1361 ff. Variations are described in European patent EP 0 581 871 B1.
Not only the surface of a sample is imaged in optical coherence tomography; in-depth imaging also takes place. While in the case of ultrasound systems a relevant signal can be assigned to a depth on the basis of certain differences in signal propagation time, in optical systems it is necessary to operate with interferometry, with the depth of the sample being examined, for example a tissue, then corresponding in terms of magnitude to the wavelength of the radiation employed.
Optical coherence tomography, which is a particularly advanced technology, enables intracorporal lumina to be imaged. For this purpose, a catheter is introduced into the body being examined. While EP 0 581 871 B1 describes an embodiment in which a bundle of fibers is provided in a catheter, with the individual fibers being addressed sequentially during scanning and not simultaneously, the present invention proceeds from an embodiment in which basically only one fiber is needed for producing a full cross-sectional image of an intracorporal lumen. Part of said fiber is therein a lens located on the distal end and a deflector unit. Light is then beamed through the fiber via the lens and the deflector unit onto the wall of the intracorporal lumen and reflected back therefrom. The reflected light is guided back through the fiber and taken to an evaluating process at the proximal end of the fiber. The cited interferometry is introduced by arranging a Michelson interferometer in front of the proximal fiber end, with the interferometer unit including a beam splitter and a reference path, with half the split beam being guided into the reference path. Said beam portion is reflected back there and traverses the reference path in the opposite direction in order to be overlaid with the signal that is decoupled from the fiber's proximal end. The thus overlaid signal is taken to an evaluation unit consisting essentially of a detector in the interferometry unit. The detector signal is taken to a signal processing system, which is to say to a computer and, where applicable, upstream electronic components. To produce a complete image, the catheter is then turned along with the fiber. The lens consequently turns along with the deflector element and the beam exiting the fiber at the distal end will be guided in another direction in keeping with the turn. Another part of the vessel wall of the lumen being imaged is irradiated analogously.
The turn is assigned in the signal evaluation unit to the signal obtained in this way so a complete two-dimensional image will result when the fiber is turned through 360°. Because different depths of the tissue of the lumen being examined are scanned simultaneously, said images will correspond to a cross-sectional image that was obtained at the height of the distal end of the fiber and provides information about the examined lumen perpendicular to the axis of rotation.
It is generally desirable for a three-dimensional data record to be generated. A plurality of said cross-sectional images must therefore be recorded at different heights of the intracorporal lumen. Because the light beam exits the fiber in a defined manner, the fiber must for this purpose be moved in the lumen. It can be moved after a cross-sectional image has been recorded, meaning after turning has taken place, so that a sequence of cross-sectional images is obtained; but it can also be moved continuously during turning, with the signals then obtained constituting a real three-dimensional data record for which the lumen was scanned spirally. The fiber is as a rule moved not forward but back. Typical speeds of retraction are around 0.5 mm/s to 2 mm/s.
What is problematic is that when blood vessels are shown in cross-sectional images the blood will disrupt the representation. Emitted light, typically having a wavelength of 1,300 nanometers, is mainly scattered by the blood's constituents. That is why in the prior art the blood is briefly kept away for image recording. That can be done by flushing the entire blood vessel with a cell-free fluid while the image is being recorded without occluding the blood flow. Another possibility is to occlude the blood flow by means of a balloon on a catheter (known from, for instance, EP 0 330 376 A2). Occluding customarily takes place upstream of the imaging site, with a little fluid being flushed into the vessel downstream so that the vessel remains cell-free and a good image quality is achieved.
At the above-mentioned speed of retraction, in the case of a three-dimensional representation of a blood vessel the problem thus arises that if a 10-centimeter length has to be represented the vessel will have to remain sealed for at least 50 seconds. That is a relatively long period for a live system.